Catalytically cracked gasoline contains considerable dienes, which would also react with other hydrocarbons in catalytically cracked gasoline, in addition to the polymerization of itself, to form precursors of coke, gums at certain reaction temperatures. Generally, these dienes can be selectively removed by hydrogenation.
U.S. Pat. No. 6,084,140 discloses a process for preparing a catalyst for the selective hydrogenation of highly unsaturated hydrocarbons in olefin fractions. Said catalyst consists of alumina and metal palladium supported on said alumina. The content of metal palladium is 0.02-3.0% by weight. Said catalyst can effectively remove the highly unsaturated hydrocarbons in olefin fractions.
U.S. Pat. No. 6,255,548 discloses a process for preparing a catalyst for the selective hydrogenation of unsaturated hydrocarbons such as acetylenes and dienes. Said catalyst is prepared by supporting at least one metal of Group VIII and at least one assistant metal M on a support. The metal of Group VIII may be one or more of nickel, palladium, platinum, rhodium, ruthenium, and iridium, preferably palladium, nickel, and platinum with a content of 0.01-50% by weight, preferably 0.05-1% by weight for noble metals and 5-30% by weight for nickel. Metal M is selected from one or more of germanium, tin, gold, and silver with a content of 0.01-10% by weight, preferably 0.02-5% by weight. Said catalyst may also contain 0.1-3% by weight of alkali metals such as potassium or sodium and/or 0.01-2% by weight of sulfur element.
U.S. Pat. No. 6,388,162 discloses a process for removing dienes from olefin feed stocks. Said process removes dienes from feedstocks with carbon numbers between 10 and 20 and rich in olefins by refining. The catalyst used in said process consists of an alumina support and metal palladium supported on said alumina support with a metal content of 0.05-0.2% by weight, preferably 0.2% by weight, wherein the specific surface area of the alumina support is no larger than 15 m2/g, preferably 2-5 m2/g.
Use of a noble metal catalyst for selective hydrogenation can effectively remove dienes from feed oils. However, the noble metal catalyst easily deactivates when used to refine full or heavy fraction of catalytically cracked gasoline containing more sulfur and nitrogen, which limits the use of said catalyst.
In general, the hydrogenation of olefins especially dienes can easily proceed under hydrorefining conditions. However, when non-noble metal hydrorefining catalysts are used in selective hydrogenation to remove dienes from feed oils, the existing problem is how to avoid or reduce coking due to polymerization of olefins and saturation of monoolefins due to hydrogenation while ensuring the removal of dienes to enhance the stability and selectivity of hydrogenation.
CN 1113829A discloses a process for preparing a hydrorefining catalyst containing molybdenum, nickel and potassium metal components, the major character of which is that the catalyst is prepared by once impregnating a pre-shaped transient state alumina including η-, γ-, and θ-alumina, preferably γ-alumina in an ammonia-containing solution of molybdenum, nickel and potassium followed by drying and calcining. The contents of the metal components in said catalyst are 10.0-24.0% of MoO3, 2.6-8.8% of NiO, 0.3-1.5% of K. Said catalyst can be used in the hydrodesulflirization of light oil fractions or gas containing organic sulfur, especially the hydrodesulfurization of straight-run gasoline and coked dry gas.
CN 1302849A discloses a protector of hydrogenation activity and its preparation, the support of which simultaneously contains γ- and δ-alumina. Said protector contains 3-22% by mole of oxides of Group VIB metals, 0.5-5% by mole of oxides of Group VIII metals, 0-2% by mole of Group IA elements, 0-3% by mole of Group VA elements and has a specific surface area of 100-250 m2/g and a pore volume of 0.4-0.8 ml/g. The oxides of Group VIB metals in said protector are oxides of Mo and W; the oxides of Group VIII metals are oxides of Fe, Co, and Ni; and the Group VA elements are P, As, Sb, and Bi. Said catalyst is suitable for use as a protector of hydrotreating catalyst for heavy oils to remove metals and gums in feed oils.
The presence of alkali metals is favorable to inhibiting coking on the catalyst surface, but this kind of catalyst has low activity when used for the hydrogenation of dienes in gasoline fraction.